This invention relates to a photoelectric sensing system. In particular, it relates to a photoelectric system arranged to detect interruptions of a light beam.
In the past, photoelectric systems utilizing light emitting and sensing devices for detecting interruptions of light beams have depended upon a variation in the total amount of light received by the sensing devices to detect the interruption of a light beam. When the light beam is broken, or the light transmission path between a light emitter and sensing device becomes sufficiently opaque, the sensing device will generate an indication of an interrupted light beam. The effects of external or ambient light on photoelectric systems also gives rise to a major disadvantage in that the amount of ambient light may be sufficient to cause the sensing device to respond at all times even in the absence of light generated by a light emitting device.
Improved types of photoelectric systems have been developed using light beams comprised of light pulse signals. Such signals utilize counters and logic circuitry to detect interruptions of the light beams. Some of these photoelectric systems have been designed to compensate for varying degrees of opaqueness of the light transmission path by manually adjusting controllers to preset the frequency and duration of transmitted light pulse signals for each transmission path environment. Other photoelectric systems utilizing beams of light pulse signals have been arranged to compensate for light transmission path variations by having a pulse stretcher circuit respond to each of various received light pulse signals. The pulse stretcher responds to each received light pulse signal by generating a signal of a fixed time duration to inhibit a detector from generating false path interruption signals. Another arrangement sometimes used by photoelectric systems has been to provide a biasing illumination source at the sensing device to improve the sensitivity of the sensing device with respect to received light pulse signals. A problem with these prior art photoelectric systems is that they are relatively complicated and require adjustment for each use in different transmission path environments.
Accordingly, a need exists in the art for a photoelectric system arranged to operate with a high degree of sensitivity over light transmission paths that may be subjected to a wide variety of light signal variations. A need also exists for a photoelectric system having relativelfy simplified apparatus capable of operating with a minimum of power and adjustment over a variety of transmission paths subject to short-term variations not constituting a transmission path interruption.